The present invention relates generally to plasma arc torches and more particularly to devices and methods for initiating a pilot arc in a plasma arc torch.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip has a relatively positive potential and operates as an anode. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, which heats and subsequently ionizes the gas. Ionized gas is then blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece because the impedance of the workpiece to ground is lower than the impedance of the torch tip to ground. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a xe2x80x9ctransferred arcxe2x80x9d mode.
One of two methods is typically used for initiating the pilot arc between the electrode and the tip. In the first method, commonly referred to as a xe2x80x9chigh frequencyxe2x80x9d or xe2x80x9chigh voltagexe2x80x9d start, a high potential is applied across the electrode and the tip sufficient to create an arc in the gap between the electrode and the tip. Accordingly, the first method is also referred to as a xe2x80x9cnon-contactxe2x80x9d start, since the electrode and the tip do not make physical contact to generate the pilot arc. In the second method, commonly referred to as a xe2x80x9ccontact start,xe2x80x9d the electrode and the tip are brought into contact and are gradually separated, thereby drawing an arc between the electrode and the tip. The contact start method thus allows an arc to be initiated at much lower potentials since the distance between the electrode and the tip is much smaller.
Plasma arc torches, including the consumable components, e.g., electrode, tip, are designed for either a contact start or a high frequency start mode. Accordingly at least one plasma arc torch and a specific set of consumables are used with a high frequency power supply, and at least one additional plasma arc torch and an additional set of consumables are used with a low voltage (contact start) power supply. As a result, for an operator that uses both high frequency and low voltage power supplies, a plurality of plasma arc torches and corresponding consumables must be purchased and maintained in inventory for continuous operations.
Accordingly, a need remains in the art to reduce the number of torches, parts, and consumables required for operation with a high frequency and a low voltage power supply. A further need exists to increase the efficiency of working with both a high frequency and a low voltage power supply.
The present invention provides a plasma arc torch that is operable with either a high frequency or a low voltage power supply, such that the torch is capable of a high frequency start or a contact start, thereby resulting in a dual mode torch. Additionally, another dual mode torch is provided that comprises a conventional contact start torch modified for operation with a high frequency power supply. Yet another dual mode torch is provided that comprises a conventional high frequency start torch modified for operation with a low voltage power supply.
In one preferred form, the present invention provides a dual mode plasma arc torch that comprises an electrode, a tip, and a start cartridge disposed between the electrode and the tip, wherein the start cartridge comprises an initiator in electrical contact with the electrode and in contact with the tip. Accordingly, when the plasma arc torch is in a contact start mode, the initiator is movable to separate from the tip and establish a pilot arc between the initiator and the tip, and when the plasma arc torch is in a high frequency start mode, the start cartridge spaces the tip from the electrode such that a pilot arc is established between the electrode and the tip.
In another form, a plasma arc torch is provided that comprises an electrode, a tip, and at least one of a contact start cartridge for a contact start mode and a high frequency start cartridge for a high frequency start mode. When the plasma arc torch is in a contact start mode, the initiator is movable to separate from the tip and establish a pilot arc between the initiator and the tip, and when the plasma arc torch is in a high frequency start mode, the high frequency start cartridge spaces the tip from the electrode such that a pilot arc is established between the electrode and the tip. Preferably, the high frequency start cartridge comprises a plurality of vent holes that provide gas flow to cool the electrode, which are offset from a center of the high frequency start cartridge in order to provide a swirling flow and further cooling capability.
In yet another form, a conventional contact start plasma arc torch is modified to comprise additional dielectric standoff, which is sized such that the contact start plasma arc torch may be operated under high frequency. Additionally, a conventional high frequency plasma arc torch is modified to comprise a movable element, e.g., electrode, tip, or third element, such that the high frequency plasma arc torch is operable under low voltage, thereby resulting in dual mode torches, i.e. torches capable of operating with either a high frequency or a low voltage power supply. Additionally, methods of operating the dual mode plasma arc torches are provided in accordance with the teachings of the present invention.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.